What is reaping?

Reaping is the process by which undesirable nodes are taken out of circulation.
When a node is reaped, NodeReaper claims the node as his own, i.e. he becomes
the new owner of the node. He places a link to a location where you can see
the old node and the reason it was considered for reaping (if one was provided).
He also informs the node's previous owner of the reapage.

A reaped node can only come back from NodeReaper's realm if it is resurrected by the gods.

A node which has been considered for reaping will be claimed by the
Reaper once all of the following conditions are met:

$reputation < 0

$reap_votes > 4

2 * $keep_votes + $edit_votes < 4

Note, however, that these conditions are only checked (and possibly acted upon) when Moderation votes are registered, not when reputation votes are cast.
Therefore, the actual reapage of a node meeting the above conditions only happens on the casting of the critical moderation reap vote, and not on the critical reputation downvote.
This is why it is possible, for example, for a node to accrue a reputation substantially lower than 0 before it is finally reaped.

Point #3 above bears some clarification. What it means is that 2 'keep' votes are sufficient to prevent a node from being reaped, as are 1 'keep' + 2 'edit', and 0 'keep' + 4 'edit'.